Shapes of Molecules

Shapes of Molecules - 14.1 14. Shapes of Molecules (text Ch...

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14.1 14. Shapes of Molecules (text Ch 5 and 7) A. Molecular Geometry Lewis structures provide us with the number and types of bonds around a central atom, as well as any NB electron pairs. They do not tell us the 3-D structure of the molecule. C H H H H CH 4 as drawn conveys no 3-D information (bonds appear like they are 90° apart) The Valence Shell Electron Pair Repulsion Theory ( VSEPR ), developed in part by Ron Gillespie at McMaster in 1957, allows us to predict 3-D shape. This important Canadian innovation is found worldwide in any intro chem course. VSEPR theory has four assumptions 1. Electrons, in pairs, are placed in the valence shell of the central atom 2. Both bonding and non-bonding (NB) pairs are included 3. Electron pairs repel each other Æ maximum separation. 4. NB pairs repel more strongly than bonding pairs, because the NB pairs are attracted to only one nucleus To be able to use VSEPR theory to predict shapes, the molecule first needs to be drawn in its Lewis structure.
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14.2 VSEPR theory uses the AXE notation (m and n are integers), where m + n = number of regions of electron density (sometimes also called number of charge clouds ). AX m E n 1. Molecules with no NB pairs and only single bonds We will first consider molecules that do not have multiple bonds nor NB pairs around the central atom (n = 0). Example: BeCl 2 o Molecule is linear (180°) Example: BF 3 o Molecule is trigonal (or triangular) planar (120°)
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14.3 Example: CH 4 o Molecule is tetrahedral (109.5°) Example: PF 5 o Molecule is trigonal bipyramidal (90° and 120°). There are three X atoms in a planar triangle and two axial atoms, one above and one below the central atom. Example: SF 6 o Molecule is octahedral (all 90°)
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14.4 2. Molecules with 1 NB pairs and only single bonds The geometry of the regions of electron density is roughly the same as what we see when no NB pairs are involved. However, the shape of the molecule is determined by looking at only the bonding pairs, NOT the non-bonding pairs.
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Shapes of Molecules - 14.1 14. Shapes of Molecules (text Ch...

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